The present invention concerns a procedure or method and a device for setting the focus and correcting errors of an electronic optical condenser of a microprojector which condenser forms a bundle of axially parallel electron rays or beams, which are projected through a large area transmission mask to create an image which is projected on a surface of a wafer by means of an electronic optical imaging system or lens system.
In the manufacture of highly integrated switching circuits, a microprojector is utilized so that an image of a large area transmission mask can be projected on the surface of the wafer, for example a monocrystalline silicon wafer as a desired reduced image. In the course of the procedure, individual structural details must be directed upon one another so as to fit in case of successively performed exposures. With progressive miniaturization, the demands on the imaging and on the illuminating system increase. Expecially critical is the proper setting of the condenser in order to get an illumination of the mask, which illumination consists of rays which are as axially parallel as possible. Also, the electron source of the microprojector must lie exactly in the front focal plane of the condenser.
By computing or measuring the focal length, the necessary distance or spacing can be approximately determined. Besides this distance, the lens aberrations and the field distortions, which occur from external influences such as magnetic materials being unsymmetrically distributed to the external axis, must be corrected by stigmators. A complete computational determination of these errors is not possible. A measurement of the condenser lens field would have to be undertaken directly in the microprojects at its operating position which measurement could only be carried out with great technical expenditures and without the required accuracy.